1. Field of the Invention
The present invention relates to a vehicle height control apparatus for controlling air pressure supplied to pneumatic shock absorbers to moderate pressure changes while the pressure is increasing and, more particularly, to an active filter that smooths rising edges of a pressure signal waveform.
2. Description of the Related Art
A motorcycle or automobile is equipped with coil spring or pneumatic shock absorbers between the body and both front and rear wheels. A conventional vehicle height control device 8 for a motorcycle, shown in FIG. 1, includes pneumatic shock absorbers 10 coupled to front forks 14 for a front wheel and rear cushions 16 for a rear wheel via pneumatic cylinders 12. Three way valves 18 are for supplying a fluid such as air to the pneumatic cylinders 12. A solenoid valve 20 is used as a front-to-rear switch and a solenoid relief valve 22 is coupled to the three way valves 18 through the solenoid valve 20. Air compressed by an air compressor 24 is supplied to the solenoid relief valve 22 through a drier 26 and the air at a desired pressure, is supplied from the solenoid relief valve 22 to the solenoid valve 20.
The solenoid relief valve 22 is equipped with a conventional pressure sensor 28 and, under instructions from a conventional control circuit 30 operated according to the outputs of the pressure sensor 28 and a vehicle height sensor 32, the pressure control action of the solenoid relief valve 22 and the switch action of the solenoid valve 20 are regulated. The control circuit 30 can be a height control computer or a discrete component control circuit such as described in U.S. Pat. Nos. 4,105,216 and 4,164,664 incorporated by reference herein. As shown in FIG. 2, the pressure sensor 28 is coupled to a conventional A/D converter 44 through a variable resistor 34 in the pressure sensor 28, the variable resistor 34 converts pressure changes into resistance changes (voltage changes). A resistor 36 and a low-pass filter circuit 38, including a resistor 40 and capacitor 42, along with the analog-to-digital (A/D) converter 44 convert the pressure changes into digital values and supply same to the control circuit 30.
The output of the pressure sensor 28, as illustrated in FIG. 3, represents the sharp changes resulting from vertical oscillation of the vehicle wheels rolling on a rugged road. The pressure sensor 28 signal waveform has large amplitude swings as shown by dot-dash line 46. The waveform is smoothed by the low-pass filter circuit 38 coupled to the pressure sensor 28, as shown by continuous line 48, resulting in a signal having relatively gentle amplitude changes.
Since the air in the pneumatic cylinder 12 is compressed in inverse proportion to the volume of the air chamber, the changes in the amplitude of the pressure show a high pressure rising ratio or steep slope as the pressure increases. Accordingly, a sharp pressure change having an acute or sharp peak is observed when the wheel has cleared a convex portion of the road surface.
The conventional vehicle height control device 8 requires amplitude control within or provided by the control circuit 30, because the output of the pressure sensor 28 fluctuates, particularly when the pressure is rising, to a greater degree than the variations in the actual road surface when the wheel is rolling on a wavy road surface. That is, the digital pressure requires smoothing to produce an average of the detected pressure. A conventional control circuit 30 is not capable of coping with rapid fluctuations in detected pressure and thus, produces inaccurate vehicle height control because of inaccurate pressure data.